Exploring the Moon's gravity: NASA's GRAIL mission

NASA's GRAIL (Gravity Recovery And Interior Laboratory) mission, with its twin spacecraft Ebb and Flow orbiting the Moon and gathering data, aims to discover more about the interior of our Moon and will use precise measurements of gravity to reveal what the inside of the Moon is like.

On 10 September 2011, NASA’s twin GRAIL spacecraft began their three month journey to the Moon.

The GRAIL twins, after reaching lunar orbit, were named Ebb and Flow as a result of a school competition. GRAIL-A was renamed Ebb, while GRAIL-B was renamed Flow. The names were chosen due to the effect that the Moon's gravity has on the Earth's tides.

Ebb (GRAIL-A) arrived at the Moon on 31 December 2011. Flow (GRAIL-B) entered lunar orbit just 25 hours later on 1 January 2012. Both craft took around 40 minutes to completely enter lunar orbit.

“Both GRAILs were launched on the same rocket but we made their trajectory to the Moon just different enough so that we could stagger their arrival times in lunar orbit,” explained GRAIL project manager David Lehmann.

The Apollo missions took only three days to travel the 400,000 kilometres to the Moon, but the GRAIL spacecraft took over three months to reach their destination. An extended journey of 4 million kilometres allowed the craft to save fuel as they gently entered lunar orbit. The relaxed pace of the GRAIL craft meant that they didn’t have to use excessive propellant in order to decelerate. The three month voyage to their lunar destination also enabled scientists to successfully test the instruments onboard the twin spacecraft.

After half a century of exploration, twelve humans have walked on the Moon and there have been over 100 missions to investigate our nearest terrestrial neighbour. But despite all this, we have still only just begun to scratch the surface in understanding the Moon. In fact, we know more about Mars than we do about our own natural satellite.

One lunar mystery that GRAIL hopes to solve is why the far side of the Moon is so different than the side we can see from Earth. “Given that we’ve sent so many missions that have studied the outside of the Moon, it seems that the answer is not on the surface,” says GRAIL principal investigator Maria Zuber. “We think that the answer is locked in the interior. GRAIL is a journey to the centre of the Moon and it will use exceedingly precise measurements of gravity to reveal what the inside of the Moon is like.”

The two GRAIL spacecraft can map how the gravity of the Moon varies in different areas. For example, the gravity will be stronger if the two craft are above a massive mountain, rather than a deep crater. The craft are also influenced by mass variations below the surface, which will allow GRAIL to investigate the interior of the Moon. Zuber explains just exactly how the two probes can measure the gravity of the Moon. “As the first spacecraft is accelerated towards a mass on the surface or the interior, it speeds up relative to the second one and the distance increases. And then as the second spacecraft comes by it catches up, so one spacecraft chases the other around the Moon and we put that information together. The change in velocity that we measure is acceleration and acceleration has units of gravity and that’s how we get the gravity field.”

The science phase of the mission began on March 7 2012.

Since arriving in lunar orbit, the spacecraft performed a series of intricate burns to achieve theh optimum orbital paths for their mission, gradually changing from an elliptical, 11.5 hour orbit to an almost circular, 2 hour orbit. This near-polar orbit means Ebb and Flow are now orbiting at an average altitude of 55 kilometres above the Moon’s surface.

"We are in a near-polar, near-circular orbit with an average altitude of about 34 miles (55 kilometers) right now," said David Lehman, GRAIL project manager from NASA's Jet Propulsion Laboratory (JPL). "During the science phase, our spacecraft will orbit the moon as high as 31 miles (51 kilometers) and as low as 10 miles (16 kilometers). They will get as close to each other as 40 miles (65 kilometers) and as far apart as 140 miles (225 kilometers)."

The science phase is expected to last 84 days, and during this time GRAIL will obtain a gravity map of the Moon with resolution that will even surpass the gravity maps of the Earth. “It will improve our knowledge of the Moon’s near side gravity by over 100 times over what was previously known and by over 1000 times over what was known on the far side,” says Zuber.

It is possible that the mission could be extended beyond June 2012 if it can survive an eclipse at the Moon. The GRAIL spacecraft are powered by solar panels, although they are also equipped with batteries. Due to the small size of the craft, there is a possibility that the batteries may not be able to sustain the craft for the duration of the eclipse. However, both GRAIL probes have performed exceptionally well on their voyage to the Moon and it they could potentially survive beyond the eclipse. If this happens, they will spiral inwards to inspect the Moon in even greater detail than before.

The extended mission would prioritise mapping shallow bowl-shaped craters on the lunar crust, which are very common features across all the terrestrial planets. Around 20 kilometres separates the bottom of the deepest crater from the top of the highest mountain. The GRAIL spacecraft would orbit at around 25 kilometres above the surface, bringing the craft very close to the mountain tops.

NASA's Jet Propulsion Laboratory (JPL) manages the GRAIL mission which is part of the Discovery Program managed at NASA's Marshall Space Flight Center. Lockheed Martin Space Systems built the spacecraft.